Protein arginine methylation is a common post-translational modification that regulates numerous cellular processes, including gene transcription, mRNA splicing, DNA repair, protein cellular localization, cell fate determination, and signaling. Three types of methyl-arginine species exist: ω NG monomethylarginine (MMA), ω NG, NG asymmetric dimethylarginine (ADMA) and ω NG, N′G symmetric dimethylarginine (SDMA). The formation of methylated arginines is catalyzed by the protein arginine methyl transferases (PRMTs) family of methyltransferases. Currently, there are nine PRMTs annotated in the human genome. The majority of these enzymes are Type I enzymes (PRMT1, -2, -3, -4, -6, -8) that are capable of mono- and asymmetric dimethylation of arginine, with S-adenosylmethionine (SAM) as the methyl donor. PRMT-5, -7 and -9 are considered to be Type II enzymes that catalyze symmetric dimethylation of arginines. Each PRMT species harbors the characteristic motifs of seven beta strand methyltransferases (Katz et al., 2003), as well as additional “double E” and “THW” sequence motifs particular to the PRMT subfamily.
PRMT5 is as a general transcriptional repressor that functions with numerous transcription factors and repressor complexes, including BRG1 and hBRM, Blimp1, and Snail. This enzyme, once recruited to a promoter, symmetrically dimethylates H3R8 and H4R3. Importantly, the H4R3 site is a major target for PRMT1 methylation (ADMA) and is generally regarded as a transcriptional activating mark. Thus, both H4R3me2s (repressive; me2s indicates SDMA modification) and H4R3me2a (active; me2a indicates ADMA modification) marks are produced in vivo. The specificity of PRMT5 for H3R8 and H4R3 can be altered by its interaction with COPR5 and this could perhaps play an important role in determining PRMT5 corepressor status.
Role of PRMTs in Cancer
Aberrant expression of PRMTs has been identified in human cancers, and PRMTs are considered to be therapeutic targets. Global analysis of histone modifications in prostate cancer has shown that the dimethylation of histone H4R3 is positively correlated with increasing grade, and these changes are predictive of clinical outcome.
PRMT5 levels have been shown to be elevated in a panel of lymphoid cancer cell lines as well as mantle cell lymphoma clinical samples. PRMT5 interacts with a number of substrates that are involved in a variety of cellular processes, including RNA processing, signal transduction, and transcriptional regulation. PRMT5 can directly modify histone H3 and H4, resulting in the repression of gene expression. PRMT5 overexpression can stimulate cell growth and induce transformation by directly repressing tumor suppressor genes. Pal et al., Mol. Cell. Biol. 2003, 7475; Pal et al. Mol. Cell. Biol. 2004, 9630; Wang et al. Mol. Cell. Biol. 2008, 6262; Chung et al. J Biol Chem 2013, 5534. In addition to its well-documented oncogenic functions in transcription and translation, the transcription factor MYC also safeguards proper pre-messenger-RNA splicing as an essential step in lymphomagenesis. Koh et al. Nature 2015, 523 7558; Hsu et al. Nature 2015 525, 384.
The discovery of cancer dependencies has the potential to inform therapeutic strategies and to identify putative drug targets. Integrating data from comprehensive genomic profiling of cancer cell lines and from functional characterization of cancer cell dependencies, it has been recently discovered that loss of the enzyme methylthioadenosine phosphorylase (MTAP) confers a selective dependence on protein arginine methyltransferase 5 (PRMT5) and its binding partner WDR77. MTAP is frequently lost due to its proximity to the commonly deleted tumor suppressor gene, CDKN2A. Cells harboring MTAP deletions possess increased intracellular concentrations of methylthioadenosine (MTA, the metabolite cleaved by MTAP). Furthermore, MTA specifically inhibits PRMT5 enzymatic activity. Administration of either MTA or a small-molecule PRMT5 inhibitor shows a preferential impairment of cell viability for MTAP-null cancer cell lines compared to isogenic MTAP-expressing counterparts. Together, these findings reveal PRMT5 as a potential vulnerability across multiple cancer lineages augmented by a common “passenger” genomic alteration.
Role of PRMT5 in Hemoglobinopathies
The developmental switch in human globin gene subtype from fetal to adult that begins at birth heralds the onset of the hemoglobinopathies, b-thalassemia and sickle cell disease (SCD). The observation that increased adult globin gene expression (in the setting of hereditary persistence of fetal hemoglobin [HPFH] mutations) significantly ameliorates the clinical severity of thalassemia and SCD has prompted the search for therapeutic strategies to reverse gamma-globin gene silencing. Central to silencing of the gamma-genes is DNA methylation, which marks critical CpG dinucleotides flanking the gene transcriptional start site in adult bone marrow erythroid cells. It has been shown that these marks are established as a consequence of recruitment of the DNA methyltransferase, DNMT3A to the gamma-promoter by the protein arginine methyltransferase PRMT5. Zhao et al. Nat Struct Mol Biol. 2009 16, 304. PRMT5-mediated methylation of histone H4R3 recruits DNMT3A, coupling histone and DNA methylation in gene silencing.
PRMT5 induces the repressive histone mark, H4R3me2s, which serves as a template for direct binding of DNMT3A, and subsequent DNA methylation. Loss of PRMT5 binding or its enzymatic activity leads to demethylation of the CpG dinucleotides and gene activation. In addition to the H4R3me2s mark and DNA methylation, PRMT5 binding to the gamma-promoter, and its enzymatic activity are essential for assembly of a multiprotein complex on the gamma-promoter, which induces a range of coordinated repressive epigenetic marks. Disruption of this complex leads to reactivation of gamma gene expression. These studies provide the basis for developing PRMT5 inhibitors as targeted therapies for thalassemia and SCD.